The invention relates to once-through steam generators and in particular to supercritical pressure steam generators which operate at subcritical pressure when at low ratings.
Steam power turbo-electric plants can be designed and operated at lower heat rates if they operate at supercritical pressures such as 220 atmospheres. The turbine is designed to pass the full steam flow with the design supercritical pressure at the turbine inlet. The steam generator must, accordingly, be designed to produce steam at supercritical pressure.
In an electric generating plant it is frequently required that the turbine operate at low load, particularly at night and during weekends when electric demand is low. At significantly reduced load where the turbine does not require supercritical pressure, continued operation of the steam generator at the high supercritical pressure actually increases the heat rate. Regardless of the turbine inlet design, there are inherent efficiency losses in such operation. Supercritical pressure steam from the steam generator must be throttled to the appropriate turbine inlet pressure. There is a substantial temperature drop involved in such throttling and, except for the few valve points on certain turbine types, there is a throttling loss. Accordingly, it would be preferred to operate the steam generator itself at a reduced pressure during the reduced load operation.
The high temperature steam turbine cannot tolerate rapid temperature changes. During a restart of the turbine it is important to match the steam temperature to the turbine metal temperature. During rapid load changes the temperature of the steam entering the turbine stages may not change drastically without creating stress damage to the turbine. As load is decreased with constant steam generator outlet temperature and pressure, the turbine valve throttling drop creates a temperature drop in the turbine.
This changing temperature limits the permissible rate of load change. If the steam generator pressure is reduced with load, the throttling pressure drop does not occur. Accordingly, the throttling temperature drop does not occur, thereby removing this limitation on the rate of load change.
For these reasons steam generators have been operated at sliding pressure. This is generally accomplished by maintaining a substantially fixed turbine throttle valve position, and varying load by changing the steam generator pressure.
With all throttle valves wide open, supercritical pressure such as 250 atmospheres, is required at full load. The required pressure decreases approximately linearly with load.
A common form of sliding pressure operation uses full pressure operation from full load down to the first valve point (about 75 to 80 percent load), and full sliding pressure below this load. This maximizes turbine efficiency of partial arc turbines, and provides energy storage for improved control response.
One of the problems, where such operation encompasses both supercritical and subcritical pressures, relates to an inherent difference in the behavior of water at supercritical and subcritical pressures. At subcritical pressures a two-phase mixture occurs in the combination of water and steam at the same temperature. At supercritical pressure the change from water to steam is gradual and uniform with no two-phase phenomenon occurring. This has created conflicting requirements on pressure part design, particularly in furnace wall circuits.
The two-phase subcritical pressure operation has the advantage that low enthalpy water may be separated from high enthalpy steam with the water being sent back for recirculation through the waterwalls. This ability to separate, however, is a problem when a two-phase mixture must be passed from one group of tubes to another, with the probability of a poor distribution of water and steam entering the circuits of the succeeding group.
Once-through boilers have been operated in the subcritical mode with a slight amount of water leaving the waterwalls. This water is then discharged back to the feedwater system. Some have been operated such that full evaporation occurs in the waterwalls and only dry steam leaves the waterwalls.
At full load operation a particular heat distribution pattern occurs in the furnace which is a function of the firing equipment used and the slagging pattern occurring on the walls. This heat absorption pattern may be predicted with reasonable accuracy. At low load operation, however, the heat distribution pattern of the same unit changes. Accordingly, designing for a predicted heat distribution at full load can create temperature maldistribution problems at low loads.
Temperature maldistribution problems show up in the steam generator as excessive temperature levels in particular tubes which are receiving heat out of proportion to the amount of flow allocated to them. They also produce excessive stresses caused by large temperature differences between various tubes in the furnace wall structure.
Several methods have been used in the past to meet this problem. The furnace wall tubes have been arranged in a spiral configuration so that they pass angularly around the entire periphery of the furnace to achieve an equalized heat absorption. This results in difficult construction problems, particularly with wall support and burner openings.
Multiple passes of fluid through the furnace wall tubes in series has been used to decrease the amount of heat absorbed in each path and, accordingly to limit the temperature unbalance occurring in each pass. This produces an arrangement with multiple downcomers which is also difficult to construct and which creates problems in distributing a two-phase mixture between the various passes.
Another approach has been to introduce mixing headers at one or more locations throughout the furnace height so that the water passing therethrough is mixed to an average heat content before entering the next section, thereby reducing temperature unbalance. This also is an expensive arrangement, and the problem of distributing a two-phase mixture continues.
The furnace wall tubes face furnace temperature on the outside and the fluid temperature on the inside. The actual tube wall metal temperature is a function of the heat transfer rate between the tube metal and the fluid passing through the tube. This heat transfer rate is generally a function of the mass flow rate of the fluid and is excellent when nucleate boiling occurs. It is good for supercritical fluid. When film boiling occurs in a plain tube, the transfer rate is very poor. It is known that internal flow disturbers such as internally ribbed tubing greatly improves this heat transfer rate where film boiling occurs.
During initial start-up of a steam generator, it is required that there be some flow through the furnace wall tubes. This is required to provide uniform heating of the structure and also to provide sufficient heat transfer at local points in the tube to avoid local overheating. On drum-type units recirculation is always used for this purpose. On once-through type steam generators a minimum flow may be provided (usually in the order of 30 percent of full load flow) with any excess beyond that which is changed to steam being passed back to the feedwater train. Another approach has been to use pumped recirculation of water through the tubes of the furnace.
The desirability of sliding pressure operation has long been known. The simplicity of fabrication and construction of a furnace structure using vertical tubes, without mixing of headers, or multiple passes is clearly desirable. Still, sliding pressure once-through units for supercritical operation have employed the complex furnace tubing arrangements.